Process for preparing chromates



United States Patent 3,403,970 PROCESS FOR PREPARING CHROMATES James B.Culbertson, Charleston, W. Va., and Headlee Lamprey, Hamden, Conn.,assignors to Union Carbide Corporation, a corporation of New York NoDrawing. Filed June 2, 1964, Ser. No. 372,121

Claims. (Cl. 23-56) ABSTRACT OF THE DISCLOSURE A process for themanufacture of chromate compounds comprises providing a mixture ofchromium bearing material (e.g., Cr O ferrochromium, chrome ore) withbarium oxide; heating the mixture in the presence of an oxygen bearinggas at a temperature in the range of 400 C. to 1000 C. to causeinter-reaction between the mixture constituents and oxygen and theformation of metal chromate; and subsequently recovering the chromatereaction product.

The present invention relates to the manufacture of chromate compounds.More particularly the present invention relates to the improvedmanufacture of chromate compounds by a novel roasting process.

Chromate compounds, e.g., Na CrO have been made in previously knownprocesses by the roasting in air of soda ash and a chromium bearingmaterial such as Cr O The chromate compounds thus obtained are usedextensively in producing sodium dichromate and in making metal-chromatepigments.

However, it has been found, using techniques of this type, that therecoverey of chromium in the form of chromate has been undesirably low,even when temperatures close to the melting point of soda ash (850 C.)have been employed. At lower temperatures e.g., 400- 500 C., there ispractically no chromate formation.

The use of lime has been proposed in place of soda ash, in view of itshigher melting point, however, the results obtained have not beensignificantly better than with the use of soda ash. Neither has the useof lime and soda ash in combination provided any substantial improvementat low temperatures.

It is therefore an object of the present invention to provide a simpleand economic chromate roasting process which provides increasedrecoveries of chromate compounds.

It is a further object of the present invention to provide a chromateroasting process which provides high chromate recoveries over a widerange of operating temperatures.

Other objects will be apparent from the following description andclaims.

A process in accordance with the present invention for the manufactureof chromate compounds comprises providing a mixture of chromium bearingmaterial with barium oxide; heating the mixture in the presence of anoxygen-bearing gas at 'a temperature in the range of 400 C. to 1000 C.to cause inter-reaction between the mixture constituents and oxygen andthe formation of metal chromate; and subsequently recovering thechromate reaction product, i.e., hexavalent chromium.

In the practice of the present invention, a chromium bearing materialsuch as Cr O is "admixed in the finely divided state with barium oxide.The amount of barium oxide ranges from 4 to 8 times, by weight, theamount of chromium as Cr, preferably from 4.4 to 5.0. The admixture canbe suitably blended using a ball mill or other similar apparatus andwhen mixing has been completed the material is introduced into asuitable roasting apparatus. Such apparatus includes 'a shaft furnace,

3,403,970 Patented Oct. 1, 1968 horizontal furnace, trays, or otherequipment in which gas can be passed through to contact the the solidsbeing heated therein.

After being charged to the roasting apparatus, the materials are heatedto between 400 C. and 1000 C. while concurrently, the materials arecontacted with oxygen bearing gas. For example, air, mixtures of oxygenwith inert gases and pure oxygen can be used. The gassolids contact canbe provided, following conventional techniques, by passing a flow of gasthrough the charge, as in a shaft furnace, or over the charge as in ahorizontal furnace. A rotary kiln can be used as the roasting apparatusin which case the oxygen-bearing gas can be passed over the heatedcharge.

In any event, regardless of the type apparatus employed, the gas contactwith the solids at temperature should be sufficient to supply at leastmole of available oxygen per mole of chromium. It has been found that amixture of argon or other inert gas with oxygen in the practice of thepresent invention is permissible but not always desirable since theinert gas will absorb heat and thus affect overall process efiiciency.

Ordinarily heating the charge materials in an atmosphere ofoxygen-bearing gas with or without admixed argon, for about 30 minutesat 450 C. will be sufficient to provide a substantially completereaction and formation of chromate. Depending upon the amount ofmaterial being" processed, and the type of apparatus used, somewhatlonger or shorter operating time will be used,

In general, the operating time for virtually complete conversion variesonly slightly for temperatures between 450 and 850 C., and tends toincrease below and above this temperature range.

The following equation is considered to theoretically represent thereaction which takes place in the practice of the present invention whenusing BaO, Cr O and oxygen as the reaction materials:

3138.0 go, 0H0. QBaCrOi BaO When the chromate-forming reaction issubstantially completed, the solid reaction product is removed from thefurnace and digested with sulfuric acid of any convenient concentration,erg. 84% to react with the produced BaCrO and provide BaSO plus CrO insolution. The resulting slurry is filtered and CrO is subsequentlyrecovered in pure form by evaporation of the filtrate.

In a particular embodiment of the present invention, a roastingtemperature of 450 C. to 650 C. is employed and it has been rfound thatexceptionally high chromate recoveries are obtained in relatively shortperiods of time.

The following examples are provided to further illustrate the presentinvention.

EXAMPLE I Cr O sized 325 mesh x D, in the amount of 1 lb. was mixed with5 lbs. of BaO powder.

The mixture was rolled together on polyethylene sheet and placed in asilica boat in a laboratory tube furnace which was sealed except for aninlet and outlet passage for the introduction and exit of gas.

The mixture was heated in the furnace to 850 C. and held at thistemperature for 30 minutes; while at this temperature a 1:1 O -argonmixture was passed over the materials in the furnace at a rate of about30 liters per minute.

At the end of the heating period, the solid reaction materials werereacted with a 20% sodium sulfate solution. The resulting slurry wasfiltered and the filtrate was evaporated and sodium chromate wasrecovered.

3,403,970 3 4 Analysis showed that 66% of the chromium in the EXAMPLEVlll Stamng matenal was recovered as chromate' The same materials andprocedure were used as in Example I except that the operatingtemperature was EXAMPLE H 650 C. The amount of chromium recovered aschromate The same procedure as in Example I was used except was 95%,that CaO was used instead of BaO. The amount of chromium recovered aschromate was 9.1%. EXAMPLE IX The same materials and procedure were usedas in EXAMPLE In Example 1 except that the operating temperature wasFerrochromium (minus 325 mesh), in the amount of 1 450 C. The amount ofchromium recovered as chromate lb. was mixed with 5 lbs. of BaO powder.was 88%.

The mixture was rolled together on polyethylene sheet The data obtainedin the foregoing examples is shown and placed in a ceramic boat in alaboratory tube furin Table I.

TABLE I Charge Tempera- Time, Cr converted Charge ratio Atmosphere ture,C. minutes to Chromate,

Percent Cr303+BaO 1:5 850 30 66 CrQO +CaO 1:5 850 30 9 1Ferroehromium+l3a0 1:3 l, 000 60 36 Ferroehromium-i-CaO 1:3 1, 000 60 5Chrome ore (ehromite)+BaO 1:5 1,000 60 36 5 Chrome ore (ehromite)+Sr0A1:5 1,000 60 CrgO +BaO 115 1,000 73 Cr2O3+Ba0 1:5 050 30 05 Cl'zO +BaO 15 450 30 88 nace which was sealed except for an inlet and outlet pas- Ascan be clearly seen from Table I a very substantial sage for theintroduction and exit of gas. improvement is obtained using BaO inaccordance with the The mixture was heated in the furnace to 1000 C. and30 Present invention as compared to the use of CaO or SrO. held at thistemperature for 1 hour; while at this tempera- It is particularlysignificant that using BaO in the ture a 1:1 O -argon mixture was passedover the materials present invention, at operating temperatures of 450C. in the furnace at a rate of about 30 liters per minute. to 650 C.,provides a remarkable improvement in At the end of the heating period,the solid reaction chromium recovery as chromate. materials were reactedwith a 20% sodium sulfate solu- In addition to chrome ore, chrome oxideand f tion. The resulting slurry was filtered and the filtrate waschromium, other chromium bearing materials can be used evaporated andsodium chromate was recovered. in the practice of the present invention,for example Analysis showed that 36% of the chromium in thestartchromium metal and chrome-silicon. i i l was recovered as chromate.The mesh slze referred to in the foregoing description are Tyler Series.EXAMPLE IV Other suitable acids and salts can be used in place of TheSame procedure as in Example III was used except shlfurrc acrfdanilhsodlurh sulfate to recover hexavalent that CaO was used instead ofBaO. The amount of chromc romlum mm 6 Teas Ion mlxture- What is claimedis: 47 1. A process for oxidizing chromium-bearing materials EXAMPLE Vwhich comprises: (1) providing a mixture of chromiumbearing materialselected from the group consisting of ChrOme Ore z a 26% 10% 15% chromeore, chrome oxide, ferro-chromium, chromium ium recovered as chromatewas 5.4%.

A1 0 2% SiO +Ca SiZed 325 mesh X In the amount metal and chrome-siliconwith barium oxide, the barium of 1 lb. was mixed Wlth 5 19 Of B30 Poxide being employed in an amount at least 4.4 times,

The mixture Was r d togfithef Polyethylene 00 by weight, the amount ofchromium as Cr in said chroplaced in a silica boat in a laboratory tubefurnace WhlCl'l miumhearing material, (2) heating the mixture in the wassealed except for an inlet and outlet passage for the presence of an ygpb i g gas Containing at least instroduction of gas. 0 percent freeoxygen, said heating being in the temperature The mixture was heat m thefurpace to 9 range of 400 c. to 1000 c. for a time sufficient to held atthis temp for P11111935; Whlle at thls cause reaction of the mixtureconstitutents and oxygen temperature a 111 g y mlxture was Passed 9 andto cause oxidation of chromium in the mixture to the materials, in thefurnace at a rate of about 30 liters to chromate and (3) Susequenflyrecovering chromate per minute. values from the mixture constituents.

At the end of the heatlflg p the Sohd react 1n 2. A process inaccordance with claim 1 wherein the materials were r act h 20% SodiumSulfate Sohmon' G0 heating is conducted in the range of 450 C. to 650 CThe resulting slurry was filtered and the filtrate was 3. A process foroxidizing chromium bearing material evaporated and sodium chromate wasrecovered. Which comprises (1) providing a mixture of crzo3 withAnalysis showed that about 36.5% of the chromium in barium Oxide, thebarium oxide being employed in an the starting material was recovered aschromate. H amount at least 44 times by weight, the amount of EXAMPLE VIchromium as Cr in said Cr O (2) heating the mixture in the presence ofan oxygen bearing gas containing at least 50 percent free oxygen, saidheating being in the temperature range of 400 C. to 1000 C. for a timesufficient to cause reaction of the mixture constituents and oxygen andto cause oxidation of chrouim in the mixture The same procedure as inExample V Were used except that SrO was used instead of BaO. The amountof chromium recovered as chromate was about 5%.

EXAMPLE VII The same materials and procedure were used as in tochromate, and (3) subsequently recovering chromate Example I except thatthe operating temperature Was values from the mixture constituents. 1000C. The amount of chromium recovered as chromate 4. A process foroxidizing chromium-bearing material was 73% which comprises: (l)providing a mixture of ferrochromium with barium oxide, the barium oxidebeing employed in an amount at least 4.4 times, by Weight, the amount ofchromium as Cr in said -ferrochr0mium, (2) heating the mixture in thepresence of an oxygen bearing gas containing at least 50 percent freeoxygen, said heating being in the temperature range of 400 C. to 1000 C.for a time sufficient to cause reaction of the mixture constituents andoxygen gas and to cause oxidation of chromium in the mixture tochromate, and (3) subsequently recovering chromate values from themixture constituents.

5. A process for oxidizing chromium-bearing material which comprises:(1) providing a mixture of chrome ore with barium oxide, the bariumoxide being employed in an amount as least 4.4 times, by weight, theamount of chromium as Cr in said chrome ore, (2) heating the mixture inthe presence of an oxygen bearing gas, containing at least 50 percentfree oxygen, said heating being in the temperature range of 400 C. to1000 C. for a time sufficient to cause reaction of the mixtureconstituents and oxygen gas and to cause oxidation of chromium in themixture of chromate, and (3) subsequently recovering chromate valuesfrom the mixture constituents.

References Cited UNITED STATES PATENTS 1,526,325 2/1965 Drefahl 23561,700,657 1/1929 Treusch 23-56 1,935,082 11/1933 Demant 2356 1,955,3264/1934 Demant 23-56 2,400,272 5/1946 Todd 2356 3,095,266 6/1963 Lauderet a1 2356 FOREIGN PATENTS 496,890 12/1938 Great Britain.

OSCAR R. VERTIZ, Primary Examiner.

B. H. LEVENSON, Assistant Examiner,

